In 1930 London, Lord and Lady Grantham, and their daughter and son-in-law, Edith and Bertie Pelham (aka Lord and Lady Hexham), attend a play starring Guy Dexter and
Noël Coward, who is also the playwright. Backstage, they meet Dexter, Coward, and former Downton Abbey butler Thomas Barrow, now Dexter's personal assistant and lover. The next day, the Crawley family, including eldest daughter Mary, attend a ball given by Bertie's cousin Lady Petersfield. When news suddenly breaks about Mary's divorce from her estranged husband, Henry Talbot, Lady Petersfield insists that Mary leave before her guests of honour,
Prince Arthur of Connaught, and
his wife arrive. Robert and Cora return to Downton Abbey; Mary remains at Grantham House to receive Cora's brother, Harold Levinson, arriving from America following their mother Martha's death. Accompanying Harold is financial adviser Gus Sambrook, who reportedly saved Harold from financial ruin from the
1929 Wall Street Crash. Sambrook and Mary are mutually attracted, drink heavily, and end up in bed together. The party travels to Downton Abbey, where Harold, to Cora's dismay, reveals that he lost their mother's fortune on poor investments during the
Great Depression. He wants to invest Downton's remaining assets to recoup his losses and repay his debt to Sambrook. Mary's divorce causes Downton neighbours to shun the Crawleys and decline a dinner invitation. Robert, still reeling from Mary's failed marriage and Harold's poor judgement, opposes Mary's suggestion to sell Grantham House in London to raise capital. When the family rejects Sambrook's proposal to invest Downton's income, he attempts to blackmail Mary regarding their liaison. Meanwhile, Tom Branson arrives with his daughter, Sybbie, in a show of support and offers to lend his father-in-law money from the sale of his business with Henry Talbot. Robert is touched by the gesture but declines. At
Royal Ascot, an acquaintance informs Tom that Sambrook is a fraudster who swindled other people's fortunes and recently avoided imprisonment. Deducing that Sambrook embezzled the Levinson wealth, Tom warns the Crawleys and Harold. Edith confronts Sambrook, forcing him to leave. Before departing, Sambrook has a final confrontation with Mary. A remorseful Harold reconciles with Cora before returning to America. Tom and Bertie Pelham convince Robert that Mary is acting in Downton Abbey's interest. He is finally able to leave Downton's management in her hands. Robert and Cora resolve to move to the Downton
dower house, taking along servants John and Anna Bates, who are expecting a second child. Anna becomes Cora's lady's maid, and Baxter becomes Mary's. Bates doubles as valet/butler. Dexter, Coward and Barrow are invited to Downton Abbey, which Edith and Cora see as an opportunity to rehabilitate Mary's reputation. They ask the servants to spread news about the impending visit in the village. Mr Molesley, now a full-time screenwriter, returns as a footman for the dinner, hoping Dexter and Coward will notice him. Molesley is disappointed when Dexter fails to remember him, though Coward is impressed that Molesley wrote
The Gambler screenplay. Molesley's spirits are lifted when, after chatting with Coward, he inadvertently provides the title for Coward's upcoming play,
Private Lives, which is inspired by Mary's divorce. The dinner is a success, and the Crawleys are again viewed favourably. Daisy Parker, now Downton's head cook following Mrs Patmore's retirement, prepares the dinner. Meanwhile, Isobel, Lady Merton, helping to organise the annual
county fair, clashes with the traditionalist chairman, Sir Hector Moreland, over how to run the event. Seeking fresh perspectives and diversity, Isobel invites Daisy and Downton's retired butler, Mr Carson, to join the committee. They support Isobel's ideas. Although Sir Hector vehemently objects to Mary as a divorcee presenting the fair's main award, Isobel successfully outmanoeuvres him; Mary presents the award to Mr Mason, Mrs Patmore's husband and Daisy's former father-in-law. Robert and Cora, the servants, and Edith express confidence and trust in Mary's ability to manage Downton Abbey. As Robert and Cora exit the Main Hall, Mary has flashbacks about her experiences in the house and deceased loved ones. Robert and Cora stroll the estate grounds as they depart for the Dower House. A mid-credits scene shows the couples in the series enjoying each other's company and Mary reading to her children. ==Cast==